Consider a client-server model based on Component Object Model (COM) technology of Microsoft. For more information, see, e.g., the Component Object Model Specification version 0.9 of October 1995 as supplied by Microsoft, herein incorporated by reference. COM is object-oriented. An object has properties that represent control functionalities of an associated electronic device as exposed to a software application. A state change of an object as a consequence of an event from outside is passed on to the software application. The application manipulates the objects by changing or setting their properties. When the application modifies property of an object associated with a certain physical device a command is sent to the associated device.
COM is a generic mechanism allowing applications to communicate in a consistent way and is a framework for developing and supporting program component objects. It provides capabilities similar to those defined in CORBA (Common Object Request Broker Architecture), the framework for the interoperation of distributed objects in a network. OLE (object linking and embedding) provides services for the compound document that users see on their display, COM provides the underlying services of interface negotiation and event services (putting one object into service as the result of an event that has happened to another object). In this implementation clients are modeled as OLE Automation objects (abstract representations) that use properties to expose controls and events to signal state changes. OLE Automation is a COM technology that enables scripting and late binding of clients to servers. OLE Automation provides communication with other programs through calls to features (commands and queries) that the programs have made available for external use. Before using an object, a client application has first to obtain the object's interface pointer. The interface pointer is obtained through the network's directory by binding the object's name or by enumerating devices. Standard COM API's for moniker binding can be used. References to objects can be obtained by calling GetObject or CoGetObject with a string specifying the desired device's name or ID. The application can then manipulate the object by setting or retrieving its properties through “set property” calls to the appropriate properties. When an application sets or modifies a property of an object corresponding with a device the property-setting operation or modification operation is converted into a command that is sent across the network to the relevant device. The objects may differ in implementation, but expose a similar property-based model to client applications running on a controller, e.g., a PC with a Windows-based operating system.